1. Field of the Invention
The present invention relates to a lightning protection fastener for use in an airframe of an aircraft, in particular, in a wing, and relates to a cap for a lightning protection fastener.
2. Description of the Related Art
In general, each of wings for constituting an airframe of an aircraft has a hollow structure, and each of wing panels forming the wing surface is secured to a structural member located inside of the wing with a fastener member (a fastener).
The fastener member includes a pin-shaped fastener main body and a fastening member (collar). The fastener member fastens the wing panel and the structural member with each other through processes in which, by inserting the fastener main body into through-holes formed in both of the wing panel and the structural member located inside the wing from the outside of the wing and by securing its tip portion by using the fastening member from the inside of the wing.
Moreover, in addition to these, with respect to the wing inner portions and the fuselage portions, structural members other than the wing panels and securing members for equipments are also fastened and secured by the fastener members.
In this case, the fastener member fastens two members through processes in which, by inserting a pin-shaped fastener main body into through-holes formed in both of the members to be secured to each other and by securing its tip portion by using a fastening member (collar).
Additionally, the wing panels or members to be secured are not limited to two.
On the other hand, in the aircraft, it is necessary to provide thoroughgoing countermeasures against a lightning strike for explosion proof. Upon occurrence of a lightning strike in an aircraft, a large current flows through a wing panel such as a main wing, or a structural member, or the like. In this case, part of or whole of the large current flows through the portion fastened by the fastener main body and the fastening member depending on cases. When the current value exceeds a limited value of a passage permissible current in each of the fastened portions, a discharge referred to as an electric arc (or a thermal spark) is generated (hereinafter, referred to as “arc” in the present specification). This is a phenomenon in which an abrupt temperature rise occurs in the fastened interface between members mainly composed of conductive members constituting the fastened portion due to a current passing through the fastened portion, with the result that the members are locally fused to cause a discharge in the air in the vicinity thereof. In most cases, scattering fused matters, referred to as “hot particles”, are generated from the fused portion. In general, since the inner space of a wing is compatibly used as a fuel tank, it is necessary to provide explosion-preventive measures. Therefore, at the time of a lightning strike, by suppressing the generation of an arc or sealing the arc, arc discharge needs to be prevented, and the scattered hot particles are prevented from contacting a combustible fuel vapor, for prevention of ignition. Here, portions at which a combustible fuel vapor might exist include the inside of each of fuel tanks in the wings and fuselage, the inside of a surge tank (a tank where a bent scoop, a burst disk, and the like are placed), which is placed on the wing tip side of the fuel tank, the inside of each of fuel system equipments, etc.
Therefore, as shown in FIGS. 5A to 5E, a structure has been proposed in which a cap 3, made of an insulating material, is attached so as to cover the tip 1a of the fastener member 1 and the metal fastener 2 fastened to the fastener member 1, and a sealant 4 is filled in the cap 3 so as to seal gaps among the tip 1a of the fastener member 1, the metal fastener 2 and the cap 3 (see Patent Literature 1 (JP-A No. 2010-254287).
In a fastening work site in which the fastener member 1 is used, as shown in FIG. 5A, upon attaching the cap 3 to the tip 1a of the fastener member 1 and the metal fastener 2, the sealant 4 is preliminarily filled into the inner space of the cap 3. Then, as shown in FIG. 5B, the cap 3, filled with the sealant 4, is mounted on the tip 1a of the fastener member 1 and the metal fastener 2. Consequently, an excessive sealant 4 inside the cap 3 spills out of an opening of the cap 3. As shown in FIG. 5C, in the case when the end face 3a on the opening side of the cap 3 is pressed onto the surface of the member 5 such as a wing panel, the spilled-out sealant 4 is present between the cap 3 and the member 5, and located on the outer circumferential side of the cap 3 in a raised ring shape, so that an arc is sealed in the interface between the cap 3 and the member 5.